This invention is directed to the manner in which a device is attached to a surface, and particularly the manner in which a circuit package is attached to a cold plate or to a circuit board, together with the facility to remove the package from the surface upon which it is mounted.
In modern electronics, integrated circuitry is formed on semiconductor chips, each of which is in turn mounted within a package for protection, connection, mounting and cooling. In some cases, a liquid adhesive is used for mounting the packages upon a base. This type of mounting relies on capillary action to cause the adhesive to run in the opening between the package and the support base on which it is mounted. This often results in non-uniform bonds and occasionally very large voids are present because of uneven flow. The attachment is not only to secure the package in place on its base, but also to conductively remove heat from the package into the base. Voids in the adhesive attachment thus reduce heat outflow from the package into the supporting base and result in inadequate cooling. In another manner of attachment, filled adhesives are placed on the base like blobs of putty, and the package is pressed into the adhesive. These materials are often avoided because of the difficulty in achieving uniformity of adhesive thickness. A uniform thickness is necessary for predictable heat flow out of the package.
When attached by either of these attachment methods, it is extremely difficult to remove a defective package. In some cases, nevertheless, repairs can be effected by removing a defective package for repair or replacement thereof. An adhesive such as silver-filled epoxy has such a high adhesion that packages are practically impossible to remove for rework without destroying the package, the printed wiring board which serves as its base, or both.
Past efforts to facilitate the removal of defective packages have concentrated on reducing the the adhesive attachment area of the bond. Such efforts necessarily reduce the heat transfer from the package to the base. In most cases, the base will be a printed circuit board carrying a plurality of packages, and thus the removal of one package so that it may be repaired and replaced or a new package substituted therefor is very helpful in containing costs. When replacement is possible, the remainder of the structure including the printed circuit board and other packages thereon may continue to be used.